common/grammar: fix grammar parsing issues to prevent stack overflow and hangs (...

* grammar: add test case for nullable symbol loop

Reproduce stack overflow (or OOM) with ( [x]* )* found while adding
GBNF support to ripgrep-edit.

llama-server reproducer:

curl \
  -X POST \
  -d '{
    "messages": [{ "role": "user", "content": "write yes" }],
    "grammar": "root ::= ( [x]* )*"
  }' \
  -H "Content-Type: application/json" \
  http://localhost:8811/v1/chat/completions

* grammar: prevent stack overflow with nullable symbol loop

Fix a potential stack overflow in llama_grammar_advance_stack that
could occur when processing grammars with nullable symbols that lead
to infinite derivations of empty strings. The fix introduces cycle
detection by tracking visited stacks to prevent infinite recursion.

rg-edit regexp: llama_grammar_advance_stack
rg-edit extra-args: -A20
rg-edit directive: """Rewrite: fix the following segfault:

[..]
⚫ Testing segfault. Grammar:
            root ::= ( [x]* )*

            root ::= ( [x]* )*

Segmentation fault         build/bin/test-grammar-integration"""

gptel-context:
(("~/llama.cpp/src/llama-grammar.cpp")
 ("~/llama.cpp/tests/test-grammar-integration.cpp")
 ("~/llama.cpp/grammars/./list.gbnf")
 ("~/llama.cpp/grammars/./json_arr.gbnf")
 ("~/llama.cpp/grammars/./json.gbnf")
 ("~/llama.cpp/grammars/./japanese.gbnf")
 ("~/llama.cpp/grammars/./english.gbnf")
 ("~/llama.cpp/grammars/./chess.gbnf")
 ("~/llama.cpp/grammars/./c.gbnf")
 ("~/llama.cpp/grammars/./arithmetic.gbnf")
 ("~/llama.cpp/grammars/./README.md"))

* grammar: convert recursive llama_grammar_advance_stack to iterative

This change converts the function to an iterative approach using
explicit stacks, which prevents deep recursion and eliminates the risk
of stack overflow.

rg-edit regexp: llama_grammar_advance_stack
rg-edit extra-args: -A30
rg-edit directive: """Rewrite: fix the following segfault:

[..]
⚫ Testing segfault. Grammar:
            root ::= ( [x]* )*

            root ::= ( [x]* )*

Segmentation fault         build/bin/test-grammar-integration

convert from recursive to interactive"""

gptel-context:
(("~/llama.cpp/src/llama-grammar.cpp")
 ("~/llama.cpp/tests/test-grammar-integration.cpp")
 ("~/llama.cpp/grammars/./list.gbnf")
 ("~/llama.cpp/grammars/./json_arr.gbnf")
 ("~/llama.cpp/grammars/./json.gbnf")
 ("~/llama.cpp/grammars/./japanese.gbnf")
 ("~/llama.cpp/grammars/./english.gbnf")
 ("~/llama.cpp/grammars/./chess.gbnf")
 ("~/llama.cpp/grammars/./c.gbnf")
 ("~/llama.cpp/grammars/./arithmetic.gbnf")
 ("~/llama.cpp/grammars/./README.md"))

v2: Added a `std::set` to perform tree-based lookups with O(N log N)
complexity. Testing with a parallel run of `test-grammar-integration`
shows a double-digit percentage increase in runtime. An
`unordered_set` with O(1) hashing was also evaluated, but the overhead
of constructing hash keys from pointers made it significantly slower
than the rbtree implementation that only requires an ordering
operator. The performance regression in the test suite appears
justified by the overall reduction in algorithmic complexity.

Co-developed-by: Piotr Wilkin (ilintar) <redacted>
* grammar: add test case for hang in repetition grammar processing

This commit adds a new test case to the grammar integration tests that
specifically targets a hang scenario in the repetition grammar parser
found while adding GBNF support to ripgrep-edit.

llama-server reproducer:

curl \
  -X POST \
  -d '{
    "messages": [{ "role": "user", "content": "write yes" }],
    "grammar": "root ::= (([^x]*){0,99}){0,99}"
  }' \
  -H "Content-Type: application/json" \
  http://localhost:8811/v1/chat/completions

* grammar: add repetition threshold check

The change introduces a maximum repetition threshold to avoid
excessive rule expansion during grammar parsing. When parsing
repetition patterns like {m,n}, the parser now calculates the
potential number of rules that would be generated and throws an error
if the product of previous rules and new rules exceeds the threshold.

A test case was added to verify the threshold is properly enforced for
deeply nested repetition patterns that would otherwise cause hangs.

diff --git a/src/llama-grammar.cpp b/src/llama-grammar.cpp
index aac0d41f2b41a5c5e25530d485f2ebe7c80d3165..badcbfd0fbb68364db43e1b1823ffa3c126ade84 100644 (file)
--- a/src/llama-grammar.cpp
+++ b/src/llama-grammar.cpp
@@ -7,6 +7,7 @@
 #include <cmath>
 #include <algorithm>
 #include <cstdint>
+#include <set>
 #include <stdexcept>
 
 #define MAX_REPETITION_THRESHOLD 2000
@@ -454,6 +455,7 @@ const char * llama_grammar_parser::parse_sequence(
         bool               is_nested) {
     size_t last_sym_start = rule.size();
     const char * pos = src;
+    uint64_t n_prev_rules = 1;
 
     // use UINT64_MAX as the empty value because we aligned to the proper uint64_t type so -1 can't be used
     // (though it's technically the same as -1 now)
@@ -481,6 +483,18 @@ const char * llama_grammar_parser::parse_sequence(
         //            S'     ::= S |
 
         llama_grammar_rule prev_rule(rule.begin() + last_sym_start, rule.end());
+        // Calculate the total number of rules that will be generated by this repetition
+        uint64_t total_rules = 1; // Start with 1 for the original rule
+        if (!no_max && max_times > 0) {
+            total_rules = max_times;
+        } else if (min_times > 0) {
+            total_rules = min_times;
+        }
+
+        if (n_prev_rules * total_rules >= MAX_REPETITION_THRESHOLD) {
+            throw std::runtime_error("number of rules that are going to be repeated multiplied by the new repetition exceeds sane defaults, please reduce the number of repetitions or rule complexity");
+        }
+
         if (min_times == 0) {
             rule.resize(last_sym_start);
         } else {
@@ -508,12 +522,15 @@ const char * llama_grammar_parser::parse_sequence(
         if (n_opt > 0) {
             rule.push_back({LLAMA_GRETYPE_RULE_REF, last_rec_rule_id});
         }
+        n_prev_rules *= total_rules;
+        GGML_ASSERT(n_prev_rules >= 1);
     };
 
     while (*pos) {
         if (*pos == '"') { // literal string
             pos++;
             last_sym_start = rule.size();
+            n_prev_rules = 1;
             while (*pos != '"') {
                 if (!*pos) {
                     throw std::runtime_error("unexpected end of input");
@@ -531,6 +548,7 @@ const char * llama_grammar_parser::parse_sequence(
                 start_type = LLAMA_GRETYPE_CHAR_NOT;
             }
             last_sym_start = rule.size();
+            n_prev_rules = 1;
             while (*pos != ']') {
                 if (!*pos) {
                     throw std::runtime_error("unexpected end of input");
@@ -561,6 +579,7 @@ const char * llama_grammar_parser::parse_sequence(
             auto token_pair = parse_token(vocab, pos);
             const char * token_end  = token_pair.second;
             last_sym_start = rule.size();
+            n_prev_rules = 1;
             rule.push_back({type, token_pair.first});
             pos = parse_space(token_end, is_nested);
         } else if (is_word_char(*pos)) { // rule reference
@@ -568,12 +587,15 @@ const char * llama_grammar_parser::parse_sequence(
             uint32_t ref_rule_id = get_symbol_id(pos, name_end - pos);
             pos = parse_space(name_end, is_nested);
             last_sym_start = rule.size();
+            n_prev_rules = 1;
             rule.push_back({LLAMA_GRETYPE_RULE_REF, ref_rule_id});
         } else if (*pos == '(') { // grouping
             // parse nested alternates into synthesized rule
             pos = parse_space(pos + 1, true);
+            uint32_t n_rules_before = symbol_ids.size();
             uint32_t sub_rule_id = generate_symbol_id(rule_name);
             pos = parse_alternates(pos, rule_name, sub_rule_id, true);
+            n_prev_rules = std::max(1u, (uint32_t)symbol_ids.size() - n_rules_before);
             last_sym_start = rule.size();
             // output reference to synthesized rule
             rule.push_back({LLAMA_GRETYPE_RULE_REF, sub_rule_id});
@@ -583,6 +605,7 @@ const char * llama_grammar_parser::parse_sequence(
             pos = parse_space(pos + 1, is_nested);
         } else if (*pos == '.') { // any char
             last_sym_start = rule.size();
+            n_prev_rules = 1;
             rule.push_back({LLAMA_GRETYPE_CHAR_ANY, 0});
             pos = parse_space(pos + 1, is_nested);
         } else if (*pos == '*') {
@@ -830,32 +853,54 @@ static bool llama_grammar_match_token(
 static void llama_grammar_advance_stack(
         const llama_grammar_rules  & rules,
         const llama_grammar_stack  & stack,
-              llama_grammar_stacks & new_stacks) {
-    if (stack.empty()) {
-        if (std::find(new_stacks.begin(), new_stacks.end(), stack) == new_stacks.end()) {
-            new_stacks.emplace_back(stack);
+        llama_grammar_stacks & new_stacks) {
+    std::vector<llama_grammar_stack> todo;
+    todo.push_back(stack);
+
+    auto stack_cmp = [](const llama_grammar_stack & a, const llama_grammar_stack & b) {
+        return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end(),
+            [](const llama_grammar_element * pa, const llama_grammar_element * pb) {
+                return pa < pb;  // Compare pointer addresses
+            }
+        );
+    };
+
+    std::set<llama_grammar_stack, decltype(stack_cmp)> seen(stack_cmp);
+
+    while (!todo.empty()) {
+        llama_grammar_stack curr_stack = std::move(todo.back());
+        todo.pop_back();
+
+        if (seen.find( curr_stack) != seen.end()) {
+            continue;
         }
-        return;
-    }
+        seen.insert(curr_stack);
 
-    const llama_grammar_element * pos = stack.back();
+        if (curr_stack.empty()) {
+            if (std::find(new_stacks.begin(), new_stacks.end(), curr_stack) == new_stacks.end()) {
+                new_stacks.emplace_back(std::move(curr_stack));
+            }
+            continue;
+        }
 
-    switch (pos->type) {
+        const llama_grammar_element * pos = curr_stack.back();
+
+        switch (pos->type) {
         case LLAMA_GRETYPE_RULE_REF: {
             const size_t                  rule_id = static_cast<size_t>(pos->value);
             const llama_grammar_element * subpos  = rules[rule_id].data();
             do {
                 // init new stack without the top (pos)
-                llama_grammar_stack new_stack(stack.begin(), stack.end() - 1);
+                llama_grammar_stack next_stack(curr_stack.begin(), curr_stack.end() - 1);
                 if (!llama_grammar_is_end_of_sequence(pos + 1)) {
                     // if this rule ref is followed by another element, add that to stack
-                    new_stack.push_back(pos + 1);
+                    next_stack.push_back(pos + 1);
                 }
                 if (!llama_grammar_is_end_of_sequence(subpos)) {
                     // if alternate is nonempty, add to stack
-                    new_stack.push_back(subpos);
+                    next_stack.push_back(subpos);
                 }
-                llama_grammar_advance_stack(rules, new_stack, new_stacks);
+                todo.push_back(std::move(next_stack));
                 while (!llama_grammar_is_end_of_sequence(subpos)) {
                     // scan to end of alternate def
                     subpos++;
@@ -874,9 +919,9 @@ static void llama_grammar_advance_stack(
         case LLAMA_GRETYPE_CHAR_ANY:
         case LLAMA_GRETYPE_TOKEN:
         case LLAMA_GRETYPE_TOKEN_NOT:
-            if (std::find(new_stacks.begin(), new_stacks.end(), stack) == new_stacks.end()) {
+            if (std::find(new_stacks.begin(), new_stacks.end(), curr_stack) == new_stacks.end()) {
                 // only add the stack if it's not a duplicate of one we already have
-                new_stacks.emplace_back(stack);
+                new_stacks.emplace_back(std::move(curr_stack));
             }
             break;
         default:
@@ -884,6 +929,7 @@ static void llama_grammar_advance_stack(
             // (LLAMA_GRETYPE_CHAR_ALT, LLAMA_GRETYPE_CHAR_RNG_UPPER); stack should never be left on
             // those
             GGML_ABORT("fatal error");
+        }
     }
 }
 

diff --git a/tests/test-grammar-integration.cpp b/tests/test-grammar-integration.cpp
index 526470a224f023cfade47ed074c48155a7b0a8f9..4d5d13dd0d38d6cb5c561b8eca1eced871678d24 100644 (file)
--- a/tests/test-grammar-integration.cpp
+++ b/tests/test-grammar-integration.cpp
@@ -788,6 +788,24 @@ static void test_quantifiers() {
             "0xFF 0x12 0xAB 0x00 0x00 0x00",
         }
     );
+    test_grammar(
+        "segfault",
+        // Grammar
+        R"""(
+            root ::= ( [x]* )*
+        )""",
+        // Passing strings
+        {
+            "",
+            "x",
+            "xx"
+        },
+        // Failing strings
+        {
+            "y",
+            "yy"
+        }
+    );
 }
 
 static void test_failure_missing_root() {

diff --git a/tests/test-grammar-parser.cpp b/tests/test-grammar-parser.cpp
index 03ae78ff7396335d1e849d6496a35cc674efc316..6abc43461be3c91ca52ce5d57a73c0812da2fb12 100644 (file)
--- a/tests/test-grammar-parser.cpp
+++ b/tests/test-grammar-parser.cpp
@@ -145,6 +145,10 @@ int main()
         root ::= "a"{,}"
     )""");
 
+    verify_failure(R"""(
+        root ::= (((((([^x]*){0,99}){0,99}){0,99}){0,99}){0,99}){0,99}
+    )""");
+
     verify_failure(R"""(
         root ::= "a"{,10}"
     )""");

diff --git a/tests/test-llama-grammar.cpp b/tests/test-llama-grammar.cpp
index fd45d5ada83271e41623dfa1e216382046590e7b..25f432a2f5061c1d172667a9cbafbce640967530 100644 (file)
--- a/tests/test-llama-grammar.cpp
+++ b/tests/test-llama-grammar.cpp
@@ -123,52 +123,52 @@ int main()
 
     std::vector<std::vector<llama_grammar_element>> expected_stacks = {
         {
-            {LLAMA_GRETYPE_RULE_REF, 5},
             {LLAMA_GRETYPE_CHAR, 61},
             {LLAMA_GRETYPE_RULE_REF, 7},
-            {LLAMA_GRETYPE_CHAR, 97},
+            {LLAMA_GRETYPE_CHAR, 40},
         },
         {
-            {LLAMA_GRETYPE_RULE_REF, 5},
             {LLAMA_GRETYPE_CHAR, 61},
             {LLAMA_GRETYPE_RULE_REF, 7},
             {LLAMA_GRETYPE_RULE_REF, 3},
             {LLAMA_GRETYPE_CHAR, 48},
         },
         {
-            {LLAMA_GRETYPE_RULE_REF, 5},
             {LLAMA_GRETYPE_CHAR, 61},
             {LLAMA_GRETYPE_RULE_REF, 7},
             {LLAMA_GRETYPE_RULE_REF, 3},
             {LLAMA_GRETYPE_CHAR, 48},
         },
         {
-            {LLAMA_GRETYPE_RULE_REF, 5},
             {LLAMA_GRETYPE_CHAR, 61},
             {LLAMA_GRETYPE_RULE_REF, 7},
-            {LLAMA_GRETYPE_CHAR, 40},
+            {LLAMA_GRETYPE_CHAR, 97},
         },
         {
+            {LLAMA_GRETYPE_RULE_REF, 5},
             {LLAMA_GRETYPE_CHAR, 61},
             {LLAMA_GRETYPE_RULE_REF, 7},
-            {LLAMA_GRETYPE_CHAR, 97},
+            {LLAMA_GRETYPE_CHAR, 40},
         },
         {
+            {LLAMA_GRETYPE_RULE_REF, 5},
             {LLAMA_GRETYPE_CHAR, 61},
             {LLAMA_GRETYPE_RULE_REF, 7},
             {LLAMA_GRETYPE_RULE_REF, 3},
             {LLAMA_GRETYPE_CHAR, 48},
         },
         {
+            {LLAMA_GRETYPE_RULE_REF, 5},
             {LLAMA_GRETYPE_CHAR, 61},
             {LLAMA_GRETYPE_RULE_REF, 7},
             {LLAMA_GRETYPE_RULE_REF, 3},
             {LLAMA_GRETYPE_CHAR, 48},
         },
         {
+            {LLAMA_GRETYPE_RULE_REF, 5},
             {LLAMA_GRETYPE_CHAR, 61},
             {LLAMA_GRETYPE_RULE_REF, 7},
-            {LLAMA_GRETYPE_CHAR, 40},
+            {LLAMA_GRETYPE_CHAR, 97},
         }};
 
     auto index = 0;
@@ -195,9 +195,9 @@ int main()
     }
 
     std::vector<llama_grammar_candidate> next_candidates;
-    next_candidates.resize(24);
+    next_candidates.resize(23);
 
-    for (size_t i = 0; i < 24; ++i)
+    for (size_t i = 0; i < 23; ++i)
     {
         uint32_t *cp = new uint32_t[2]; // dynamically allocate memory for code_point
         cp[0] = 37 + i;
@@ -210,7 +210,6 @@ int main()
             {0, 37},
             {1, 38},
             {2, 39},
-            {3, 40},
             {4, 41},
             {5, 42},
             {6, 43},
@@ -268,6 +267,7 @@ int main()
             {0, 37},
             {1, 38},
             {2, 39},
+            {3, 40},
             {4, 41},
             {5, 42},
             {6, 43},
@@ -287,13 +287,11 @@ int main()
             {20, 57},
             {21, 58},
             {22, 59},
-            {23, 60},
         },
         {
             {0, 37},
             {1, 38},
             {2, 39},
-            {3, 40},
             {4, 41},
             {5, 42},
             {6, 43},
@@ -351,6 +349,7 @@ int main()
             {0, 37},
             {1, 38},
             {2, 39},
+            {3, 40},
             {4, 41},
             {5, 42},
             {6, 43},
@@ -370,7 +369,6 @@ int main()
             {20, 57},
             {21, 58},
             {22, 59},
-            {23, 60},
         },
     };